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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Claims Development [Line Items]        
Prior year claims and claims adjustment expense $ 27,499,000 $ (4,690,000) $ (301,000)  
Increase of unpaid losses and loss adjustment expenses 189,931,000 (40,346,000) (35,513,000)  
Unpaid losses and loss adjustment expenses 248,425,000 58,494,000 98,840,000 $ 134,353,000
Reinsurance recoverable $ 182,405,000 106,000 $ 13,540,000 $ 47,350,000
Insurance claim maximum settlement time 1 year      
Initial case reserve for claims $ 2,500      
LAE case reserves 0      
Subrogation [Member]        
Claims Development [Line Items]        
Reserve for losses and loss adjustment expenses $ 85,000,000 $ 76,000,000